1. Field of the Invention
The invention relates to a discharge lamp in which the ends of the discharge vessel are sealed with a material with a gradient function. One end of this material with a gradient function is electrically conductive and the other end is dielectrical. On one end, a dielectric component prevails, and proceeding in the direction toward the other end, the electrically conductivity continuously or incrementally increases until it is electrically conductive.
2. Description of the Related Art
In a discharge lamp within a spherical or ellipsoidal arc tube, in the middle of the quartz glass discharge vessel, there is a pair of opposed electrodes, and an emission metal, such as mercury or the like, and a discharge gas and the like are encapsulated. Cylindrical sealing tubes are joined to opposite ends of the arc tube, and upholding parts of the electrodes and outer lead pins are sealed, in an electrically connected state, by these sealing tubes. However, sealing cannot be achieved by direct welding of the sealing tubes to the upholding parts of the electrodes, since the molybdenum upholding parts of the electrodes and the quartz glass sealing tubes have very different coefficients of thermal expansion. Therefore, the sealing tubes were conventionally sealed by a step joining process, a foil sealing process, or the like.
In the step joining process, intermediate glass tubes of various types are prepared; their coefficients of thermal expansion proceed from the coefficient of thermal expansion of quartz glass, incrementally, until they approach the coefficient of thermal expansion of tungsten. These intermediate glass tubes are welded in stages proceeding from the two ends of the sealing tubes of the discharge vessel, as a result of which the length of the sealing tubes is increased. The glass tubes on the ends with a coefficient of thermal expansion nearest the coefficient of thermal expansion of tungsten are welded to the upholding parts of the electrodes. If fewer intermediate glass tubes are used, the differences between the coefficient of thermal expansion of the adjacent intermediate glass tubes become greater; this causes low mechanical strength at the joining sites, low resistance to thermal shocks, and a reduction of reliability. It is therefore necessary to increase the number of intermediate glass tubes.
Furthermore, at a high temperature, e.g., greater than or equal to 400.degree. C., in luminous operation, oxidation of tungsten occurs because the tungsten rods and the glass ends are in contact with air. Here, there is the danger of leakage and breakage. The length of the sealing tubes in the axial direction is increased and multiple joining sites are formed, so that reliability is reduced accordingly.
In the foil sealing process, the ends of the upholding parts of the electrodes and the outer lead pins are welded to the two of molybdenum foils having a thickness of a few dozen microns. These molybdenum foils are clamped between quartz glasses, and sealing tubes of quartz glass are welded to the center area of the molybdenum foils. In this foil sealing process, at a high temperature of greater than or equal to 350.degree. C. in luminous operation, oxidation of the molybdenum foils occurs because the ends of the molybdenum foils to which the outer lead pins are welded are in contact with air. Here, a leak occurs due to loosening of the seal areas as a result of expansion by oxidation or breakage. This means that, in the seal areas on the ends of the sealing tubes, the temperature increase must be suppressed. Therefore, it necessary to lengthen the sealing tubes, and thus, the distance between the seal areas and the arc tubes whose temperature during luminous operation increases.
In the case of a discharge lamp using mercury vapor, when the distance between the arc tube and the seal areas is increased, the tube wall temperature drops on the base points of the upholding parts of the electrodes. This means that the temperature of the coolest part within the arc tube becomes too low, and the mercury does not vaporize sufficiently. Therefore, it becomes necessary to provide the outside of the tube wall at the base points of the upholding parts of the electrodes with a heat insulating film and thus to effect heat insulation. But here, it is considered disadvantageous that the light is shielded by this heat insulating film and that the lighting efficiency decreases.
By means of the step joining process or foil sealing process, the sealing tubes of the discharge lamp are lengthened in the axial direction in this way. In a light irradiation device in which one of the sealing tubes of a discharge lamp of the short arc type is installed in the center opening of a concave reflector, and in which another sealing tube extends in the direction of the optical axis of the concave reflector, there is however the disadvantage of a decrease in the efficiency of the light due to the long length of the sealing tube, which extends in the direction of the optical axis of the concave reflector, so that some of the light reflected by the concave reflector is incident on this sealing part and shielded.
Recently, a discharge lamp has become more and more important in which the sealing tubes on the ends of the discharge vessel are sealed by sealing bodies which are made of a material with a gradient function which is formed from a dielectric powder, such as silicon dioxide, and a conductive powder, such as molybdenum. In a sealing body which is made of a material of this type with a gradient function, one end is rich in the dielectric component, such as silicon dioxide, while in the direction to the other end, the ratio of the conductive component, such as molybdenum, increases continuously or in stages. Therefore, the vicinity of one end of the sealing body is dielectric and has a coefficient of thermal expansion which approaches the coefficient of thermal expansion of quartz glass. The vicinity of the other end is conductive and has the property that its coefficient of thermal expansion approaches the coefficient of thermal expansion of molybdenum.
For this material with a gradient function, it is possible to increase the gradient with which the ratio of the dielectric component to the conductive component changes. The sealing body made of the material with the gradient function can, therefore, have the dielectric material in a considerable amount on one end and the conductive material in a considerable amount on other end, even if the length is relatively short in the axial direction. Furthermore, the material with the gradient function has no interface on which the composition of its material components changes significantly. Therefore, here, the resistance to thermal shocks and the mechanical strength are great.
Therefore, the seal areas in which the sealing bodies are welded to the sealing tubes can also approach the arc tube which reaches a high temperature in luminous operation. The advantages arise that the sealing bodies have a short length in the axial direction and that, furthermore, the sealing tubes are made shorter. Therefore, the above described disadvantages in the foil sealing process and the step joining process can be eliminated.
As is shown in FIG. 2, conventionally, at the sealing of sealing tubes 12, joined at the opposite ends of an arc tube 11 of a discharge vessel 10, by means of sealing bodies 50 which are made of a material with a gradient function, end faces 51 of sealing bodies 50, which contains a dielectric component, such as silicon dioxide, in a considerable amount, are directed toward the arc tube 11 and sealing bodies 50 are inserted into sealing tubes 12. The neighboring areas of faces 51 are heated and sealing bodies 50 are welded to sealing tubes 12. Furthermore, openings are provided proceeding from the opposite inner and outer end faces 51, 52 of sealing bodies 50 to the areas of electrical conductively. An upholding part 21 of the anode 20 and an upholding part 31 of cathode 30 are inserted and attached in the respective openings of inner end faces 51, and an anode terminal 22 and a cathode terminal 32 are inserted and attached in the respective opening of the respective outer face 52. In this way, the upholding part 21 is electrically connected to anode terminal 22 and the upholding part 31 to cathode terminal 32. The upholding part 21 and the upholding part 31 are, furthermore, often attached by means of solder 60 in sealing bodies 50.
Here, as was described above, sealing tubes 12 are shortened and sealing bodies 50 are brought near to arc tube 11, which reaches a high temperature in luminous operation. The sites attached by means of the solder 60, therefore, are often corroded by plasmas which form during discharge or by discharge gas with an increased temperature. There are cases in which impurity gases and the like form in these attachment sites and have an adverse effect on the lamp characteristics, or shorten the lamp service life.
Furthermore, in a discharge lamp, especially one of the short arc type, it is necessary to exactly fix the positions of the electrodes and to exactly control the distance between the electrodes. If the sealing bodies in which the upholding parts of the electrodes are attached are welded to the sealing tubes, it is, however, difficult to exactly fix the positions of the sealing bodies. Therefore, the disadvantage arises that the positions of the electrodes are often inexact.
In a sodium high pressure lamp, in an arc tube made of transparent aluminum oxide, sealing bodies of aluminum oxide are inserted which are penetrated by power supply wires of niobium which are hermetically sealed to the outer faces of the sealing bodies by sealing glass. In a lamp of this type, there is no process in which the glass is flame-welded. Therefore, the above described disadvantage that the positions of the electrodes are often inexact does not arise.